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Physics 221, January 19
Key Concepts:
•Scalar Quantities and Vector Quantities
•Position, Distance, Displacement
•Average Speed, Average Velocity
•Instantaneous Speed, Instantaneous Velocity
•Average Acceleration, Instantaneous Acceleration
Electronic Devices
Please separate your professional from your social life
Do not use social media during class.
Physics: the key word is change!
Matter interacts and the interactions change the physical state of matter.
The laws of physics predict when and how the physical state of matter changes.
Mechanics:
Change < -- > Motion
The laws of mechanics predict when and how things move.
We have to agree on a way to describe motion.
Vector quantities with magnitude and direction are used in describing motion:
Position:
r = ai + bj + ck
Displacement:
d = (x2 - x1)i + (y2 - y1)j + (z2 -z 1)k
Average velocity:
v = d/∆t.
Instantaneous velocity:
Let ∆t  0.
Average acceleration: a = ∆v/∆t.
Instantaneous acceleration:
Let ∆t  0.
Scalar quantity with only magnitude are also used in describing motion:
Examples: distance, average speed, instantaneous speed
Extra Credit: Four different mice (labeled A, B, C, and D) ran the triangular maze
shown below. They started in the lower left corner and followed the paths of the
arrows. The times they took are shown below each figure,
(i) Which mouse had the greatest average speed?
(ii) Which mouse had the greatest average velocity?
1.
(i) D, (ii) C
2.
3.
4.
(i) B, (ii) A
(i) A, (ii) D
(i) B, (ii) C
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Motion in 1 dimension
If an object is restricted to move in one dimension, for exampe along the xaxis, we can specify the vector quantities
position, displacement, velocity, and acceleration
by a signed number with units. The sign of the number than specifies the
direction.
In one dimension, if the x-component of a vector is positive, the vector is
pointing in the positive x-direction, and if the x-component of a vector is
negative, the vector is pointing in the negative x-direction.
We can represent one-dimensional motion using a position versus time
graph or a velocity versus time graph.
The position versus time graph
represents the motion of an
object moving in a straight line.
Find the objects instantaneous
velocity at
(a) t=0.5s, (b) t=2.0s, (c) t=4.0s.
1.
2.
3.
4.
5.
2 m/s, (b) 5 m/s, (c) 6 m/s
2 m/s, (b) 8 m/s, (c) 10 m/s
0 m/s, (b) 8/3 m/s, (c) 2 m/s
0 m/s, (b) 3 m/s, (c) 1 m/s
0 m/s, (b) 3/2 m/s, (c) 1/2 m/s
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The position versus time graph
represents the motion of an object
moving in a straight line. Find the
average velocity of the object
between
(a) A and C, (b) A and D, (c) B and D.
1.
2.
3.
4.
5.
2 m/s, (b) 8/5 m/s, (c) 2 m/s
8/3 m/s, (b) 2 m/s, (c) 5/2 m/s
4 m/s, (b) 5 m/s, (c) 6 m/s
8 m/s, (b) 10 m/s, (c) 10 m/s
5 m/s, (b) 6 m/s, (c) 6 m/s
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The position versus time graph
represents the motion of an object
moving in a straight line. Is the
magnitude of the average velocity
equal to the average speed between
A and D?
1.
2.
Yes
No
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The position versus time graph
represents the motion of an object
moving along a straight line. The
velocity of the object is always
positive.
1.
2.
True
False
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The position versus time graph
represents the motion of an object
moving along a straight line. What is
the instantaneous velocity of the
object at t = 14 s?
1.
2.
3.
4.
5.
-(1/15) m/s
-(5/3) m/s
0.067 m/s
(1/3) m/s
-(2/3) m/s
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A friend is casually talking with you about physics concepts and brings up the
following claim. “They say that average speed for a trip is total distance
divided by total time, which is true. Then they say that if the trip consists of a
person traveling at two different speeds for two different times then you can
get the average speed by just finding the average speeds for the two parts of
the trip and then take their average.” Is their claim true or false?
What is your opinion?
1. True
2. False
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The velocity versus time graph to
the right represents the motion of
an object moving in one dimension
along the x-axis.
The velocity of the object is always
positive.
1. True
2. False
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Extra Credit: The velocity versus
time graph to the right represents
the motion of an object moving in
one dimension.
What is the object's average
acceleration between t = 0 and t =
6s?
1.
2.
3.
4.
5.
-3 m/s2
1.5 m/s2
-0.83 m/s2
0.67 m/s2
0.5 m/s2
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The positions of two blocks at successive 0.20-second time intervals are represented by the
numbered squares in the figure below. The blocks are moving towards the right.
The accelerations of the blocks are related as follows:
1.
2.
3.
4.
5.
The acceleration of a is greater than the
acceleration of b.
The acceleration of a equals the acceleration
of b. Both accelerations are greater than
zero.
The acceleration of b is greater than the
acceleration of a.
The acceleration of a equals the acceleration
of b. Both accelerations are zero.
Not enough information is given to answer
the question.
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Let the x-axis of your coordinate system point along a drag strip. A
drag racer starts from rest and accelerates to a speed of 76.5 m/s in
8.5 seconds. What is the acceleration of the car?
1.
2.
3.
4.
5.
2.1 m/s2
68 m/s2
9 m/s2
650 m/s2
76.5 m/s2
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Extra Credit: A 50-g superball traveling at 25.0 m/s bounces off a
brick wall and rebounds at 22.0 m/s. A high speed camera
records this event. If the ball is in contact with the wall for 3.5
10-3 s, what is the magnitude of the average acceleration during
this time interval?
1.
2.
3.
4.
5.
13429 m/s2
857 m/s2
47 m/s2
3 m/s2
0.01 m/s2
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